Tomorrow I pick up my 30g tank. It's going to be planted....heavily planet. I don't plan on having any substrate visible, driftwood and rocks - covered with moss and other plants. So I know that to keep all that up and running, I'm going to need a lot. I plan on doing a soil based substrate, so I need to know if I'm going to need a co2 setup. I currently have a 10g tank with a dozen plants, and about six fish (I don't like to overstock or even push it).

Basically, can you tell me when CO2 is absolutely necessary? I've run planted tanks, but nothing as heavy as what I'm going to venture into.

Welcome to the forum!
The short answer is no, CO2 is not a requirement for a planted tank. Adding CO2, which may be a limiting factor in an aquatic system, just accelerates the rate of plant growth up to 2000%, but also increases maintenance and nutrient consumption. Please read Byron's eloquent "stickies" regarding a planted tank. Balancing light, CO2, and nutrients is required. Increase one, and you may need to balance the increase out with additions from the other limiting factors.

Nope unless you are planning a high tech setup, 6 fish in a 10 gallon will be plenty to supply a decent amount of co2.

In my 30 I'll probably end up running two shoals (species not determined yet) of ~10 fish and some shrimp (ghost cherry or amano, possibly bamboo if I feel like going that big but that's unlikely) for starters. I just don't want to start planting my tank and having product that I recently purchase die the more I plant, because there's not enough key nutrients to go around. Every other tank I've ever had aside from my 10 has been fake plants. I'm at the point where I'll never go back to that again, either :P

Welcome to the forum!
The short answer is no, CO2 is not a requirement for a planted tank. Adding CO2, which may be a limiting factor in an aquatic system, just accelerates the rate of plant growth up to 2000%, but also increases maintenance and nutrient consumption. Please read Byron's eloquent "stickies" regarding a planted tank. Balancing light, CO2, and nutrients is required. Increase one, and you may need top balance the increase out with additions from the other limiting factors.

Any particular reason you are using a soil substrate?

I read the stickies twice in the past week, it makes sense but I still wanted extra opinions on the matter. And no particular reason, I've read several threads (I've been lurking the forums since before I made my account and began posting, lots of nice theorycrafting and ideas floatin around) about the soil substrate and it seems like a new and fun idea to tackle. And I plan on slowly adding plants and bulking up the flora in my tank before going nuts with the fauna, so what better time to start playing with a different substrate.

I've not personally done a soil substrate because there is no benefit long-term that I have ever found reading those who have; but having read numerous soil-substrate methods one thing to note is that the tank should be planted and heavily at day one. The dry method seems less troublesome, though it means running the tank for 6 months before fish are introduced. In other words, this isn't a substrate that allows you to set up a tank in a day, such as you can easily do with any other substrate and plants with fish added at the same time.

And on the CO2, in addition to what DKRST pointed out: The soil-substrate methods do not use CO2 diffusion since this defeats the purpose of the soil substrate which is there to encourage CO2 via organic breakdown in the substrate. My thinking would be that with both, you would be more liable to really have an organic soup.

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian.[unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]